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Due to the increased bacterial resistance to common antibiotics, there is tendency towards using herbal extractsin order to increase the non-specific immune system. This study was conducted to evaluate the effects of different levels of Mentha piperita extract on hematological, biochemical and immune parameters of Oncorhynchus mykiss. For this purpose, fish with mean (±SD) weight of 32.2± 0.12 g were raised for 8 weeks in Vniro tanks (1800 l water), 80 fish to each tank and feeding with different levels of M. piperita (with concentrations of % 0, % 1, % 2.0 and % 3.0, three replicates were used for each concentration). At the end of the trial, blood samples were collected to determine some hematological, biochemical and immunity parameters in different groups and compared to one another. Results showed significant differences in RBC and WBC count, neutrophil percentage, haemoglobin (Hb), haematocrit (HCT) value, total protein (TP), IgM and lysozyme activity in fish fed M. piperita (especially with 2 and 3.0% concentrations) when compared with control group. The results suggest that M. piperita extract may enhance the non-specific immune system of O.mykiss. Thus, using this supplement especially at of 3.0% level as immunostimulants was recommended in O.mykiss diet.